In recent years, transmission methods in an optical transmission system by which asynchronous client data input from a plurality of client apparatuses is converted into frames and then the frames are multiplexed and transmitted are examined. Such transmission methods include, for example, the SONET (Synchronous Optical Network) system, SDH (Synchronous Digital Hierarchy) system, Ethernet (registered trademark) system, and OTN (Optical Transport Network) system. Among these transmission systems, the OTN system receiving particular attention transmits client data of the SONET system or SDH system transparently.
In the OTN system, when client data is multiplexed, the client data may be arranged in TS (Tributary Slot) on the network side in units of OTN frames or may be arranged in TS on the network side after the client data being demultiplexed into a plurality of pieces of data of a lower layer. Also in the OTN system, client data may directly be returned to a client apparatus. Thus, a cross connection function is required in the optical transmission system.
Japanese Laid-open Patent Publication No. 2003-188919
However, according to conventional technology, a problem of a growing apparatus configuration is posed. According to conventional technology, when asynchronous client data is multiplexed on the client side, a PLL (Phase Locked Loop) circuit may be used at least for each port because cross connections occur asynchronously. Such a PLL circuit may be used for the purpose of suppressing jitter accumulation due to interference of asynchronous client data. Also according to conventional technology, when a frame signal is demultiplexed on the network side, the PLL circuit may be used for each port because the clock is extracted for each frame and cross connections occur asynchronously. As a result of the above circumstances, according to conventional technology, the PLL circuit may be used at least for each port on the client side or the network side, leading to a growing apparatus configuration.